The Ardunio community is also building a small form factor STM32F405 board in same form factor as the famous blue Pill (stm32f103c8t6) here is a pic of it https://ibb.co/m512be. They have https://robotdyn.com/ to manufacture it for them but they r wanting a very noise free ADC so r still in the testing stage of the ADC and I am not sure when/if it will ever be available. They are aiming for $10US plus postage price range.

A STM32F405 offers a lot more then the STM32f103 but all the cheap STM32F405 boards don't have the same form factor. I have a STM32F407VET that I prototype on but it is a very big for a robot control board.

I am keen to get one of the nano's on this thread if the postage isn't a killer on the price.

Maybe you should check out the design the Ardunio guys have been working on as they have designed a STM3232F405RG board same size and pin compatible as a blue-pill with a SD card slot on the bottom of the board and then 4 solder pads for the remaining pins that didn't fit on the bread board breakout being pin compatible with blue pill. see the super long thread http://www.stm32duino.com/viewtopic.php?f=19&t=2633